This invention relates to television receivers capable of varying the aspect ratio of the reproduced image and, more particularly, to television receivers with which the aspect ratio can be automatically switched.
While with the standard broadcast television signals the aspect ratio of the reproduction on the picture tube is set to 3:4, with closed television systems and video tape recorders for special household and other private purposes it is possible to freely select a desired aspect ratio independently of the standard system for non-standard television signals can be used in these systems; for instance it is possible to obtain a wide image compared to the standard system image by setting the aspect ratio to 1:2.
In this caes, an identification (ID) signal S.sub.I at a particular frequency may be superimposed on the sync chip portion of the sync signal S.sub.S of a non-standard television signal as shown in FIG. 2, and the television receiver side may be arranged such that the deflection circuit thereof may be automatically switched with detection of the ID signal. Designated at S.sub.B in FIG. 2 is the signal.
FIG. 1A shows the block diagram of the prior art television receiver in which the aspect ratio of the image reproduction on the picture tube can be automatically changed. With this construction, when receiving the standard broadcast television signal, the composite video signal derived from the television intercepted by the antenna and supplied to the input terminal 1, is coupled to a video signal amplifier 2 for amplifying the video signal in the input signal, the amplified video signal being coupled to a cathode-ray tube 3, while the input signal is also coupled through an identification signal elimination circuit 4 and a deflection circuit 5 to produce a deflection current flowing through a horizontal deflection yoke 8. In the deflection circuit 5, a sync separator, an oscillator, an output circuit, etc. are included. At this time, since the standard television signal contains no ID signal, it is coupled to the deflection circuit 5 without any change of the sync signal. Also, since the input signal is coupled through a band-pass filter 6 and an ID signal detection circuit 7, but since no ID signal is contained, the detected output is zero, and image reproduction with the ordinary aspect ratio of 3:4 is reproduced is obtained in the area labeled A of the cathode-ray tube 3. This image reproduction in the area A of the cathode-ray tube 3 in FIG. 1B is focused by a lens 9 onto a wide screen 10 in an area A' thereof as shown in FIG. 1C.
When receiving a non-standard television signal, which contains the ID signal S.sub.I inserted in the sync signal S.sub.S, the ID signal is separated from the rest of the television signal by the band-pass amplifier 6 and rectified in a predetermined way by the ID signal detecting circuit 7, the output of which is supplied to the deflection circuit 5. The amplitude of the horizontal sawtooth wave of the output of the deflection circuit 5 is automatically controlled by the output of the ID signal detecting circuit 7, whereby a corresponding aspect ratio of the image reproduction on the cathode-ray tube 3 (for instance of 1:2) is obtained.
More particularly, an elongated image, for instance, is reproduced in the areas A, B.sub.1 and B.sub.2 is reproduced as shown in FIG. 1B, and its projection is focused on the screen 10 over the entire areas A', B.sub.1 ' and B.sub.2 ' as shown in FIG. 1C.
The areas C.sub.1 and C.sub.2 of the cathode-ray tube 3 are not used.
The signal which contains the ID signal S.sub.I superimposed upon the sync signal S.sub.S, as used with the prior art system described above, can be used only in a closed system such as a VTR for private purpose and is not suitable for the standard systems prescribed by the broadcasting standards. In addition, it is impossible to provide sufficient amplitude or duration of the ID signal S.sub.I to ensure steady and reliable detection of this signal, and therefore mis-operation is prone.
Aside from the aforementioned ID signal, it has also been proposed to insert an ID signal in a fixed one horizontal scanning line period in the vertical blanking period like vertical interval reference signal and detect it. Such an ID signal, however, is restricted for use by the broadcasting standards. In addition, in the case of the 1/2-inch size VTR the aforementioned ID signal in the vertical blanking period cannot be accurately reproduced and is liable to be dropped out at the time of playback.
Further, it has been proposed to insert and detect a continuous wave at a frequency at the end of or outside the audible frequency range for the purpose of switching the aspect ratio. However, in the case of the household VTR, the frequency of this ID signal has to be set within the audible frequency range, thus leading to objectionable reproduction of sound.